Electrostatic spray gun

ABSTRACT

An electrostatic spray system operable to spray either electrically conductive materials or nonconductive materials. The system includes apparatus for mechanically atomizing the coating material and for electrostatically charging it prior to deposition onto an article, which apparatus is characterized by an improved spray gun having improved electrical isolation between the electrically conductive portions of the gun and the nonconductive portions. Specifically, it includes an improved connector for connecting the gun to a supply hose without having to permanently weld the two together to maintain an effective electrical &#39;&#39;&#39;&#39;standoff&#39;&#39;&#39;&#39; between the hose and the gun. It also has an improved cartridge for sealing the valve stem of the gun. The system of this invention also includes an improved material storage tank and insulative system for isolating that tank from ground when the system is used to spray electrically conductive materials. It also includes a grounding wire for connecting the tank to ground when the tank is used to spray nonconductive materials.

Unitedfitates Patent 11 1 Hastings et al.

[451 July 24, 1973 ELECTROSTATIC SPRAY GUN [73] Assignee: Nordson Corporation, Amherst,

Ohio

[22] Filed: Nov. 16, 1971 [21] Appl. No.: 199,114

52 U.S. c1. 239/3, 239/15 51 I B05b 5/02 58 Field 01 Search 239/3, 15; 118/629 [56] References Cited UNITED STATES PATENTS [57] ABSTRACT An electrostatic spray system operable to spray either electrically conductive materials or nonconductive materials. The system includes apparatus for mechanically atomizing the coating material and for electrostatically charging it prior to deposition onto an article, which apparatus is characterized by an improved spray gun having improved electrical isolation between the electrically conductive portions of the gun and the nonconductive portions. Specifically, it includes an improved connector for connecting the gun to a supply hose without having to permanently weld the two together to maintain an effective electrical standoff between the hose and the gun. It also has an improved cartridge for sealing the valve stem of the gun. The system of this invention also includes an improved material storage tank and insulative system for isolating that tank from ground when the system is used to spray electrically 3,635,400 l/l972 Nord et al.... 239 15 3,621,815 11/1971 Wanm-g 239/15] x conductlve materials. It also 1ncludes a grounding wire 3,645,447 211972 Cowan 239/15 for connecting the tank to ground when the tank is used to spray nonconductive materials. Primary Examiner-M. Henson Wood, Jr. Assistant Examiner-Michael Mar 16 Claims, 4 Drawing Figures Attorney-James S. Hight I J6 [j 22 83 /g 3 :4 42 v v y g 4 4 4 .58 d7 J6 i( 57 6/3 j A m. a

75 I .s/ .53 7 2 7/ 15 J2 63 46 5/ 47 93 74 73 37 if a I //6 w .22 /J w 7; /ze

PATENTEDJULZMHB SHEU 2 OF 3 INVENTORS 2.

ELECTROSTATIC SPRAY GUN This invention relates to spray systems and particularly to electrostatic spray systems.

Until recently very nearly all electrostatic spray equipment was designed primarily if not exclusively for use with electrically nonconductive paints or sprays. The spraying of electrically conductive paints, as for example, water base paints, was generally considered to be potentially too dangerous to permit its use in electrostatic systems because such paints had to be used in isolated systems which created shock hazards to the operating personnel. Now, though, there is a great deal of interest in water base paints and the electrostatic spraying of them because these paints contain no atmosphere polluting solvents of the type toward which a great deal of public criticism is now being directed.

The capability of spraying both nonconductive and conductive materials in electrostatic spraying equipment has always been highly desirable and has even in some instances been practiced. However, when electrostatic systems have been used to spray electrically conductive materials, such as water base paints, the results have been less than optimal, primarily because of excessive shunting of current to the grounded parts of the system, especially to the grounded spray gun handle, with the consequent decrease in voltage at a charging electrode at the forward end of the spray gun.

Another objective of this invention has been to provide a better sealed and better electrically insulated electrostatic spray gun than has heretofore been available. Most hand manipulated or manually operated electrostatic spray guns have a valve at the front end of the gun which controls the passage of liquid through the nozzle located adjacent the valve by actuation of a handle located at the rear of the gun. A control rod extends from the handle to the valve. in the case of electrostatic spray guns, this rod, or at least a 3-inch section of it, is made of dielectric material (as for example, one of the common plastics). The dielectric portion of the rod is generally at least 3 inches in length so that there is a 3-inch electrical standoff along the surface of the rod from the electrically conductive portion of the gun to the nonconductive or grounded handle portion of the gun. The 3-inch standoff is considered the minimum required for safe utilization of equipment over which the electrical charge could pass via dirt or other contaminants on the surface of the standoff passage. One of the materials which commonly dirties or contaminates the standoff surface is the liquid material being applied by the spray equipment.

To protect the 3-inch standoff area of the rod against contamination by the spray-material, there is usually a static seal located around the rod. One form of static seal which is commonly used for this application is a bellows type seal which has one end fixedly secured to the valvc stem and an opposite end fixedly secured to the dielectric body within which the stem is movable. Exampes of patents which disclose the use of this type of seal are Shaffer U.S. Pat. No. 3,583,632 and Beach U.S. Pat. No. 3,339,841. While these seals have been found to be very adequate against the ingress of paint or other liquid spray material into and over the surface of the dielectric portion of the pull rod from the front of the gun, they do not protect against the ingress of contaminants from the rear of the gun. We have found and another aspect of this invention is predicated upon the determination that one source of loss of the 3-inch electrical standoff along the surface of the pull rod occurs as a result of leakage of solvent material from the rear or from the handle portion of the gun forwardly along the valve stem. This leakage occurs when the gun is placed in a bucket of solvent material (as commonly happens between shifts in a production facility). To combat this reverse leakage of liquid contaminants forwardly along the valve stem, we provide a second static type bellows seal adjacent the rear end of the rod with the two seals being separated by approximately a 3- inch electrical standoff. This double bellows arrangement has the effect of assuring that liquid contaminant cannot enter and dirty the standoff surface from either end of the pull rod. For ease of assembly the double bellows and the pull rod as well as the surrounding and separating dielectric spacer or sleeve and a portion of the valve and a spring for biasing the valve to a closed position are all made as a cartridge assembly which may be easily inserted and removed from the gun.

Another aspect of this invention is also predicated upon the maintenance of a 3-inch electrical standoff or an equivalent seal between an electrically conductive portion of the gun and an exposed surface of the gun. One of the common areas where it is difficult to maintain the 3-inch standoff when the gun is used to spray electrically conductive materials is in the hose connection between the dielectric or nonconductive paint or spray hose and the dielectric barrel of the gun. This problem is compounded by the necessity for using a double, coaxial paint conduit. This is due to the fact that most materials available for the paint hose are porous to some extent and become saturated by the water or solvent component of the paint, and hence would provide another ready path of shunt current to ground if made up by only a single layer. Because of the diffrculties in maintaining the 3-inch standoff at this point, it is common to weld the hose to'the gun, usually by a spin welding technique. Such a connection is hydraulically and electrically satisfactory but it is also extremely rigid and prone to easy breakage. And when such a connection is damaged, since it is welded and not removable, a major portion of the gun must be removed and a time-consuming replacement made.

We have found that this premanent connection of the hose to the gun is unnecessary and may be avoided while still maintaining the same nonconductive electrical properties of the connection by forming tapered pipe threads on the dielectric hose fitting and on the nipple for receiving the hose fitting into the gun. When the tapered pipe threads are joined, the two dielectric elements are forced together and coupled so well that the two act as a single solid piece of plastic or dielectric material insofar as the electrical conductivity through the surface between the two. This pipe threaded connection has the advantage of eliminating the necessity for a 3-inch standoff and of avoiding the practice of permanently welding the two parts together. We also provide a novel means for securing the two members of the coaxial paint conduit to the hose fitting while maintaining the necessary electrical standoff at these junctions also.

It has also been discovered that there is excessively rapid deterioration of resistors in the spray gun when electrostatic spray equipment is used to spray water base or electrically conductive sprays. It is well known that when electrostatic spray equipment is used to spray nonconductive materials, the paint reservoir or holding tank must be grounded so as to prevent an electrical charge from slowly leaking back through the paint or spray to the reservoir and building up to a dangerous potential. When the equipment is used to spray electrically conductive paints or sprays, the full electrical charge passes directly through the paint back to the reservoir so that the reservoir is at full electrical potential and must be maintained isolated from ground to avoid short circuiting or shunting the current through the paint conduit and reservoir. We have found that when the paint reservoir is isolated from ground as it must be to spray conductive materials, shunt current is caused to flow through a corona effect at all of the corners of the reservoir when the tank is clean. As soon as the tank becomes dirty there is an improved path over the surface of the container via the contaminants for the current to shunt to ground. This increased flow of shunt current from the reservoir causes an increased current flow through the resistor which is not sufficient to drop out a safety relay in the electrical circuit but which is sufficiently high to slowly but steadily deteriorate and destroy the resistor.

To correct this condition and eliminate this excessively rapid deterioration of the resistor when the equipment is used to spray electrically conductive materials, the invention of this application incorporates a casing of dielectric material around the tank or reservoir of spray material. This casing prevents the current from shunting to atmosphere around the tank through a corona discharge when the tank is clean and/or the greater current flow to ground through contaminants when the tank is dirty. When the tank and equipment are used to spray electrically nonconductive materials, this dielectric cover around the tank is bypassed and the tank is grounded by removing a section of the dielectric covering and connecting the electrically conductive portion of the tank directly to ground.

The primary advantage of this electrostatic spray system is that it may be used for spraying both electrically conductive and nonconductive materials with equal facility. This system has the advantage of being safer in use either when spraying electrically conductive or nonconductive materials than systems which have preceded it because of the better protected standoff distance along the surface of the pull rod. Additionally, the gun has the capability of being disassembled from the spray hose for purposes of cleaning and repairing it without sacrificing any safety or creating a potential hazard to its use.

These and other objects and advantages of this invention will be more readily apparent from the following description of the drawings in which:

FIG. 1 is an exploded side elevational view ofa manually operated electrostatic air spray gun incorporating the invention of this application;

FIG. 2 is an end elevational view of the gun of FIG. 1.

FIG. 3 is a cross sectional view taken on line 3-3 of FIG. 2;

FIG. 4 is a cross sectional view taken on line 4-4 of FIG. 2.

The gun illustrated in the drawings is an air operated electrostatic spray gun which relies upon the impact of an air stream with a liquid stream to effect atomization of the liquid stream. While the invention is described as applied to an air gun, it should be understood, though, that the invention of this application is equally applicable to all electrostatic spray guns or spray systems.

The gun 10 comprises an electrically conductive metal handle assembly 11, an electrically insulative barrel assembly 12, and an insulative nozzle assembly 13. Paint or other spray material, which may be in the nature of a coating or a varnish or a lacquer (referred to in regard to this invention generically as paint) is supplied to the gun from a reservoir or tank 14. A high voltage source of electrical energy is supplied to the gun from an electrical power pack 15.

The handle assembly 11 is generally made from a metal casting and includes an air inlet 16, a trigger actuated air flow control valve 17, and a trigger 18 for controlling the flow of air through the valve 17. There is also an adjustable air valve 20 in the gun handle for controlling the shape or fan" of the spray emitted from the gun, as is explained more fully hereinafter.

The air inlet port 16 opens into a generally vertical air passage 21 which communicates with a transverse counterbored air valve passage 22. The air passage 22 in turn communicates with a pair of air lines 23, 24 which in turn communicate with passages 25, 26 through the barrel 27 of the gun.

The air valve 17 which controls air flow from the inlet passage 21 to the barrel passage 25, 26 comprises a trigger actuated pin 28, a pin retainer 30, a plunger 31, and a plunger limiting adjustable stop 32. One end of the pin 28 abuts the trigger 18 while the opposite end of the pin is receivable in a central aperture of the plunger 31. The retainer 30 is threaded into the handle and serves as a guide for the pin. An O-ring seal (not shown) between the pin 28 and the retainer 30 precludes leakage of air from the air passage 22. A compression spring 33 is located between the plunger 31 and the stop 32. This spring biases a resilient seal 34 on the end of the plunger against a shoulder of the passage 22 so as to maintain the air valve 17 in a normally closed position.

The fan control valve 20 comprises a single valve plunger which is threaded into the counterbored threaded end of the passage 23. Its end is tapered and is engageable with a tapered seat machined into the air passage 23 to limit or close air flow through the passage.

The barrel assembly 12 is made from an electrically insulative material such as one of the common plastics and includes the main body section 27 through which the pair of air lines or passages 25, 26 extend, as well as a material flow control passage 35 and an electrical flow control passage 36. The material flow control passage 35 is intersected by an inclined passage 37 through which material is supplied to the passage 35 from a flow control pipe 38. The opposite or lower end of this pipe 38 is supported from the handle 11 by a bracket 40.

As may be seen most clearly in FIG. 4, air flow in the passage 26 of the barrel is controlled by the trigger actuated valve 17 while air flow in the passage 25 is regulated by the valve 20 of the handle. At the forward end of the barrel body 27, a passage (not shown) through the nozzle assembly communicates between the air flow passage 26 and the passage 41 of the nozzle assembly 13. This latter passage 41 is located between a fluid nozzle 42 and an air nozzle 43. It is open at the front so that it defines an annular air passage 44 around the material orifice (not shown) of the fluid nozzle 42. Air

issuing from this air passage 44 impacts with the stream of material issuing from the material orifice of the nozzle 42 and at least coarsely atomizes the stream. There may be additional ports of the air nozzle 43 connected with the passage 26 to further atomize the stream. There are also a pair of fan shaping ports (not shown) located in a pair of horns 45 of the air nozzle 43 which communicate through a passage of the air nozzle 43 with the passage 25 of the barrel 27. Adjustment of the valve 20 controls the amount of flow of air issuing from the horns of the nozzle and thus the degree of fan formed by the atomized spray.

Flow of fluid material through the material or fluid nozzle 42 is controlled by a valve 50 located at the forward end of the passage 35. This valve comprises a metal seat 47 located in a counterbored recess 48 and a movable needle valve 50. At its forward end, the needle valve 50 has a needle 51 receivable in an axial aperture of the seat 47 to close the valve and preclude the passage of the material into and through the nozzle assembly13. The movable needle valve 50 forms the end portion of a cartridge assembly 52. At its opposite end this cartridge assembly is attached to the trigger 18 so that in addition to controlling the flow of air through the gun, the trigger controls the flow of material through the gun. The connection of the trigger to the air valve 17 and a retainer stem 53 of the cartridge assembly 52 is such that the air valve 17 always opens and initiates air flow before the material flow control valve 46 opens.

The valve control cartridge 52 comprises the movable needle valve'50, a stem assembly which consists of a pair of studs 54, 55 interconnected by a guide 56, and a surrounding seal which comprises a pair of bellows seals 57 and 58-spaced apart by a spacer sleeve 60. Additionally, the cartridge includes a compression spring 61 located between the flange 60' on spacer sleeve 60 and the flange 54' on stud 54. The stud 54, the bellows seals 57 and 58, the guide 56, and the spacer sleeve 60 are all made from electrically nonconductive materials.

The forward end of the stud 54 is threaded into a threaded aperture in the rear of the needle valve block 46. Adjacent the threaded end flange 54' cooperates with a shoulder 62 at the rear of the needle valve 50 to lock or securely fix one end 63 of the bellows seal 57 to the valve stem. The opposite end of this bellows seal 57 is permanently attached to the forward end of the sleeve 60 and sealed with wall of passage 35 by an O- ring seal 64.

At its rearward end, the forwardmost stud 54 is threaded into the slidable guide 56. This guide as well as the spacer sleeve 60 within which it is slidable are both made from electrically nonconductive materials so as to provide an electrical standoff between the two. At its rearward end this guide 56 has a threaded aperture which receives the threaded forward end of the stud 55. The rearward end of this same stud is connected to the trigger 18 by the trigger retainer 53 which has a necked-in section 66 received within a vertical slot of the trigger 18. Thus, rearward movement of the trigger causes the retainer 53, the stud 55 and the inner connective elements 54 and 56 of the valve stem to move rearwardly with it and pull the needle valve 50 to an open position against the bias of the spring 61.

The rearward end or shoulder 67 of the guide 56 cooperates with a shoulder 68 on the forward end of the stud 55 to lock the forward end 70 of the bellows seal 58 against movement relative to the valve stem assembly. The rearward end 71 of this bellows seal 58 is forced by a rear shoulder 72 of the spacer sleeve 60 against the forwardmost flat surface or end surface 73 of the gun handle 11 so that the rear end of the bellows seal 58 is fixed against movement relative to the body 27.

All of the components of the valve control cartridge 52 are made from insulative material except for the needle 51, the spring 61, the rearwardmost stud 55, and the trigger retainer 53. Consequently, the nonconductive components of the cartridge maintain an electrical standoff between the electrically conductive elements at the forward end of the gun and the metal conductive elements, and particularly the handle, at the rear of the gun. Also, reduction of conductive components in contact with the paint reduces the capacity of the gun for build-up of capacitive energy which could arc to ignite the volatile atmosphere or shock the operator, thereby minimizing other safety hazards.

By fixing the forward end of both bellows seals to the valve stem and the rearward end of the seals against movement relative to the body 27, a static seal is formed by each of the bellows seals at one end of the valve stem. Theforwardmost bellows functions to prevent paint or contaminants from moving rearwardly over the valve stem assembly and, similarly, the rearwardmost bellows functions to prevent contaminants from moving forwardly over the valve stem assembly from the area in the vicinity of the trigger. Frequently, the guns are placed in a bucket of solvent when not is use or between work shifts so as to prevent the paint from hardening in the gun and clogging the conduits. In the absence of a bellows seal 58 adjacent the rearward end of the valve stem, the solvent may enter and flow forwardly along the stem and thereby provide an electrical flow path which completely destroys the electrical standoff between the electrically conductive and nonconductive elements of the gun.

The material nozzle 42 is made from an electrically nonconductive material which is threaded into a counterbore 74 in the forward end of the body 27. It has an axial passage or bore (not shown) which opens into the rear of the counterbore 74. The rear of the counterbore 74 in turn communicates with the central aperture of the valve seat 47 via a passage 75 such that material passing through the aperture of the valve seat 47 may enter and pass through the axial passage in the material nozzle 42. The axial passage in the nozzle 42 terminates in a small diameter outlet for discharge of a solid stream or jet of coating material.

The air nozzle 43 is also made from an electrically nonconductive material. It is threaded over a threaded sector of the barrel and has air passages which connect the ports in the horns 45 to the fan air control passage 25 of the barrel.

Liquid or fluid is supplied to the material passage 35 of the gun via the inclined passage 37 from the storage tank or reservoir 14. This tank is connected via an electrically nonconductive fluid conduit 76 to the electrically nonconductive coaxial flow control pipe 38 which terminates into a non-conductive hose fitting 77. The hose fitting 77 has tapered pipe threads 78 machined onto its forward end which are received into tapered pipe threads formed on the interior of the passage 37. Pipe threads, when viewed in cross section are V- shaped as opposed to machine threads which generally have the bottom of the V removed. Because the pipe threads are tapered from one end to the other and are V-shaped in cross section when joined, the two plastic nonconductive pipe threaded elements of the fitting form a near perfect fluid seal which, insofar as electrical conductivity through the seal is concerned, has approximately the same dielectric characteristics as if the two units of the coupling were made of a single piece of nonconductive material. This pipe threaded coupling thus eliminates the necessity to spin weld the hose fitting to the nonconductive body of the gun as has heretofore been a conventional practice.

The inner paint hose 171 of coaxial conduit 38 is inserted through a nonconductive lock fitting 80 into internal receiving opening 172, and rests with its forward surface against shoulder 173 of hose fitting 77. By screwing lock fitting 80 tightly into threads 176 on the interior of fitting 70, ferrule 174 is wedged tightly between fitting 70 and inner hose 171 to hydraulically seal the junction. O ring seal 175 seals the bottom end of fitting 77 against the entry of contaminants and the necessary standoff distance to minimize the flow of shunt current between fitting 77 and lock fitting 80 is provided by the helical path around the mating threads 176.

Outer hose 177 of coaxial conduit 38 enters the bottom of lock fitting 80, seats with its forward surface against shoulder 178 and is sealed against the entry of contaminants by O-ring seal 179. Outer hose 177 is thrust forward into lock fitting 80 and held tightly in place by the ferrule 180 of bracket fitting 181 which is screw threaded to mating fitting 182 to also secure the coaxial conduit to bracket 40. A clearance of at least 0.015 inch is maintained between the inner and outer hoses 171 and 177 respectively to ensure that even if the inner conduit becomes saturated by the water or solvent from the paint there will still be sufficient dielectric strength in the air gap and the unsaturated outer hose to prevent current from shunting through the side wall of the coaxial conduit.

Electrical power is supplied to an antenna 81 which protrudes from the orifice of the fluid nozzle 42. This power is supplied from the electrical power pack which is connected to the antenna via an insulated cable 82, an aluminum washer 83, a spring 84, and washer 85, through a resistor 86 and an electrical lead 87, a washer 88 and spring 90. The end of the spring 90 terminates in the antenna 81.

The cable 82 is a conventional flexible coaxial cable within which there is a central electrical conductor surrounded by an insulating sheath, a conductive grounding sheath, and an encasing sheath of insulation. This cable is secured to'the gun by a conventional locking plug 9]. The electrically conductive portion of the cable together with its electrically insulative sheath extends upwardly from the plug into and through a nonconductive hose 92. This hose fits within the handle of the gun and protrudes from the forward end. It receives the aluminum washer 83 which makes contact with the electrical conductor in the cable as well as the spring 84, the washer 85 and the resistor 86. The electrical conductor 87 is embedded in the nonconductive barrel 27 of the gun and carries the electrical charge from the end of the resistor to the end of the spring 90. The spring 90 is surrounded by a nonconductive antenna holder 93 which fits into the fluid nozzle 42. At its forward end the antenna portion 81 of the spring 90 extends from the antenna holder 93 and out through the fluid orifice at the end of the nozzle 42.

This system may be used to spray either electrically conductive or nonconductive paints or sprays. When the system is used with electrically conductive paints or sprays, as for example, water base paints, the paint reservoir 14 is maintained isolated from ground by insulative mountings 94, 95 and by the removal of a grounding lead 96. When isolated in this manner the electrical current is prevented from shunting back through the electrically conductive material to the tank or reservoir and then dissipated to ground through the corners of the tank. An electrically insulative material or casing surrounds the tank and prevents corona discharge from the tank as well as preventing the tank from becoming dirty and then leaking current to ground through the dirt or contaminant. This dielectric insulation thus prevents a slow but steady current drain through the paint reservoir and consequent deterioration of the resistor 86.

When the system is used to spray nonconductive materials a threaded plug 98 in the casing 97 is removed and a new plug 100 inserted. This new plug 100 has a spring-biased probe extending through it so that it establishes good electrical contact with the electrically conductive reservoir 14 contained within the casing 97. The opposite end of the probe is secured to the grounding wire 96. Thus when used to spray a nonconductive material, the grounding wire prevents current from shunting rearwardly through the paint in the paint conduit and building up capacitive charge on the reservoir. This charge if allowed to build over a prolonged period of time without a grounding wire on the reservoir can become very dangerous because of the reservoir acting as a capacitor and storing a high voltage charge.

One advantage of the system described hereinafter is that it is readily interchangeable between spraying electrically conductive and nonconductive sprays. Additionally, it has the advantage of being easily assembled and disassembled for purposes of cleaning and repairing it. The fluid control cartridge including the double bellows arrangement and the insulative shield around it as a separate subassembly facilitates assembly and disassembly. Additionally, the use of non-welded components in the connection of the electrically nonconductive coaxial hose to the nonconductive body facilitates assembly and disassembly.

While we have described only a single preferred embodiment of our invention, persons skilled in the art to which this invention pertains will readily appreciate numerous changes and modifications which may be made without departing from the spirit of our invention. Therefore, we do not intend to be limited except by the scope of the following appended claims.

Having described our invention, we claim:

1. An electrostatic spray gun having an electrically an "electrically nonconductive conduit adapted to connect said reservoir to said body passage, and

the improvement which comprises a removable valve control cartridge mounted on said body, said cartridge including said movable valve closure element and a valve stem extending rearwardly from said closure element,

an electrically nonconductive sleeve surrounding said stem, and

a pair of static seals located'at opposite ends of said sleeve, each of said seals being fixedly secured at one point against movement relative to said stern and at another point against movement relative to said body.

2. The spray gun of claim 1 in which said cartridge further includes a compressible spring located between said movable valve closure element and one of said static seals.

3. An electrostatic spray gun having an electrically nonconductive body,

a material passage in said body terminating in an outlet orifice,

a valve for. opening and closing said orifice including a stem having an electrically nonconductive section extending into said passage for controlling opening and closing of said valve,

a first static sealing means located in such passage adjacent said valve and surrounding said stem, said sealing means being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body so as to prevent the flow of material rearwardly over said stem,

means for applying an electrical charge to material as it is ejected from said orifice,

electrically conductive'connector means adapted to connect said antenna to a high voltage power sup- I ply, and

a second static sealing means located in said passage surrounding said stern, said second sealing means being fixedly secured at one point against movement relative to said stern and at another point against movement relative to said body, said second sealing means being spaced rearwardly from said first sealing means and operable to prevent the ingress of contaminants forwardly along said stem.

4. The spray gun of claim 3 in which said valve includes a stationary valve seat and a movable. valve element, said movable valve element being secured to said valve stem and movable therewith, and a spring located between said movable valve element and one of said static seals for biasing said valve to a closed position.

5. The spray gun of claim 4 in which said movable valve clement, said spring, said pair of static seals and at least a portion of said movable valve stem are all intcrconnected in a separate subassembly cartridge, which subassmebly cartridge may be removed and reassembled as a unit in said body. 1

6. The spray gun of claim'5 in which. said subassembly cartridge further comprises a spacer sleeve located between and separating said pair of static seals, and a guide element separating and interconnecting two sec tions of said valve stem,said guide element being slideable in said spacer sleeve.

7. The spray gun of claim 6 in which said spacer sleeve and guide are both'made from electrically nonconductive materials.

8. An electrostatic spray system including a spray gun having an electrically nonconductive body,

a material passage in said body terminating in an outlet orifice,

a valve for opening and closing said orifice,

electrically conductive paint charging means extending through said orifice,

electrically conductive connector means connecting said paint charging means to a high voltage power pp y a material storage reservoir,

an electrically nonconductive conduit extending from said reservoir to said body passage, and

the improvement which comprises a nonconductive connector for connecting said conduit to said body so as to establish communication between body passage and a passage through said conduit, said connector comprising a pair of threaded elements on said conduit and said body respectively, both of said threaded elements having tapered pipe threads formed thereon so that when secured together, said joined threaded elements have approximately the same electrical conductivity characteristic of a solid piece of said nonconductive material.

9. The spray system of claim 8 in which said valve comprises a stationary valve seat, a movable valve element and a valve stem connected to said valve element, said system further comprising a pair of spaced static seals surrounding said stem, each of said seals being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body.

.10. The spray system of claim 9 which further includes a spring located between said movable valve element and one of said static seals for biasing said valve to a closed position.

11. The spray system of claim 10 in which said movable valve element, said spring, said pair of static seals and at least a portion of said movable valve stem are all inter-connected in a separate subassembly cartridge, which subassembly cartridge may be removed and reassembled as a unit in said body.

12. The spray system of claim 11 in which said subassembly cartridge further comprises a spacer sleeve located between and separating said pair of static seals, and a guide element separating and interconnecting two sections of said valve stem, said guide element being slideable in said spacer sleeve.

13. The spray system of claim 12 in which said spacer sleeve and guide are both made from electrically nonconductive materials.

14. In an electrostatic spray system of the type which includes a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, an electrically conductive storage tank for supplying material to said passage, a valve for opening and closing said outlet orifice, electrical circuit means for applying an electrical charge to material as it is ejected from the orifice, and an electrical resistor in said circuit,

the method of retarding breakdown and failure of the electrical resistor which method comprises encasing said electrically conductive storage tank in an electrically nonconductive housing to prevent electrical discharge to atmosphere from the surface of said storage tank when said system is utilized to spray electrically conductive materials. 15. An electrostatic spray system comprising a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, an electrically conductive storage tank for supplying material to said passage, a valve for opening and closing said outlet orifice, electrical circuit means for applying an electrical charge to material as it is ejected from the orifice, and an electrical resistor in said circult,

ing either an electrically conductive material or a nonconductive material including a spray gun having an electrically nonconductive body,

a material passage in said body terminating in an outlet orifice,

a valve for opening and closing said orifice including a stem having an electrically nonconductive section extending into said passage for controlling opening and closing of said valve,

sealing means located in such passage and surrounding said stem,

means for applying an electrical charge to material as it is ejected from said orifice,

an electrical resistor,

first electrically conductive connector means connecting one end of said resistor to said electrical charge applying means,

second electrically conductive ocnnector means connecting said resistor to a high voltage power supply,

a material storage reservoir,

an electrically nonconductive conduit connecting said reservoir to said body passage, and

means to prevent the failure of said resistor when said system is used to spray electrically conductive materials, said resistor failure preventing means comprising electrically insulative mounting means for said reservoir,

electrically nonconductive means surrounding and encasing said reservoir to prevent electrical discharge of said reservoir when said system is utilized to spray electrically conductive materials, and

means to ground said reservoir when said system is used to spray electrically non-conductive materials. 

1. An electrostatic spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, a valve for opening and closing said orifice, said valve comprising a stationary element mounted in said body and a movable closure element, electrically conductive paint charging means adjacent said orifice, electrically conductive connector means adapted to connect said paint charging means to a high voltage power supply, an electrically nonconductive conduit adapted to connect said reservoir to said body passage, and the improvement which comprises a removable valve control cartridge mounted on said body, said cartridge including said movable valve closure element and a valve stem extending rearwardly from said closure element, an electrically nonconductive sleeve surrounding said stem, and a pair of static seals located at opposite ends of said sleeve, each of said seals being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body.
 2. The spray gun of claim 1 in which said cartridge further includes a compressible spring located between said movable valve closure element and one of said static seals.
 3. An electrostatic spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, a valve for opening and closing said orifice including a stem having an electrically nonconductive section extending into said passage for controlling opening and closing of said valve, a first static sealing means located in such passage adjacent said valve and surrounding said stem, said sealing means being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body so as to prevent the flow of material rearwardly over said stem, means for applying an electrical charge to material as it is ejected from said orifice, electrically conductive connector means adapted to connect said antenna to a high voltage power supply, and a second static sealing means located in said passage surrounding said stem, said second sealing means being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body, said second sealing means being spaced rearwardly from said first sealing means and operable to prevent the ingress of contaminants forwardly along said stem.
 4. The spray gun of claim 3 in which said valve includes a stationary valve seat and a movable valve element, said movable valve element being secured to said valve stem and movable therewith, and a spring located between said movable valve element and one of said static seals for biasing said valve to a closed position.
 5. The spray gun of claim 4 in which said movable valve element, said spring, said pair of static seals and at least a portion of said movable valve stem are all interconnected in a separate subassembly cartridge, which subassmebly cartridge may be removed and reassembled as a unit in said body.
 6. The spray gun of claim 5 in which said subassembly cartridge further comprises a spacer sleeve located between and separating said pair of static seals, and a guide element separating and interconnecting two sections of said valve stem, said guide element being slideable in said spacer sleeve.
 7. The spray gun of claim 6 in which said spacer sleeve and guide are both made from electrically nonconductive materials.
 8. An electrostatic spray system including a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, a valve for opening and closing said orifice, electrically conductive paint charging means extending through said orifice, electrically conductive connector means connecting said paint charging means to a high voltage power supply, a material storage reservoir, an electrically nonconductive conduit extending from said reservoir to said body passage, and the improvement which comprises a nonconductive connector for connecting said conduit to said body so as to establish communication between body passage and a passage through said conduit, said connector comprising a pair of threaded elements on said conduit and said body respectively, both of said threaded elements having tapered pipe threads formed thereon so that when secured together, said joined threaded elements have approximately the same electrical conductivity characteristic of a solid piece of said nonconductive material.
 9. The spray system of claim 8 in which said valve comprises a stationary valve seat, a movable valve element and a valve stem connected to said valve element, said system further comprising a pair of spaced static seals surrounding said stem, each of said seals being fixedly secured at one point against movement relative to said stem and at another point against movement relative to said body.
 10. The spray system of claim 9 which further includes a spring located between said movable valve element and one of said static seals for biasing said valve to a closed position.
 11. The spray system of claim 10 in which said movable valve element, said spring, said pair of static seals and at least a portion of said movable valve stem are all inter-connected in a separate subassembly cartridge, which subassembly cartridge may be removed and reassembled as a unit in said body.
 12. The spray system of claim 11 in which said subassembly cartridge further comprises a spacer sleeve located between and separating said pair of static seals, and a guide element separating and interconnecting two sections of said valve stem, said guide element being slideable in said spacer sleeve.
 13. The spray system of claim 12 in which said spacer sleeve and guide are both made from electrically nonconductive materials.
 14. In an electrostatic spray system of the type which includes a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, an electrically conductive storage tank for supplying material to said passage, a valve for opening and closing said outlet orifice, electrical circuit means for applying an electrical charge to material as it is ejected from the orifice, and an electrical resistor in said circuit, the method of retarding breakdown and failure of the electrical resistor which method comprises encasing said electrically conductive storage tank in an electrically nonconductive housing to prevent electrical discharge to atmosphere from the surface of said storage tank when said sysTem is utilized to spray electrically conductive materials.
 15. An electrostatic spray system comprising a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, an electrically conductive storage tank for supplying material to said passage, a valve for opening and closing said outlet orifice, electrical circuit means for applying an electrical charge to material as it is ejected from the orifice, and an electrical resistor in said circuit, the improvement which comprises means to prevent breakdown and failure of the electrical resistor when the system is utilized to spray electrically conductive materials, which resistor breakdown and failure preventing means comprises an electrically nonconductive housing surrounding and encasing said electrically conductive storage tank to prevent electrical discharge to atmosphere from the surface of said storage tank.
 16. An electrostatic spray system adapted for spraying either an electrically conductive material or a nonconductive material including a spray gun having an electrically nonconductive body, a material passage in said body terminating in an outlet orifice, a valve for opening and closing said orifice including a stem having an electrically nonconductive section extending into said passage for controlling opening and closing of said valve, sealing means located in such passage and surrounding said stem, means for applying an electrical charge to material as it is ejected from said orifice, an electrical resistor, first electrically conductive connector means connecting one end of said resistor to said electrical charge applying means, second electrically conductive ocnnector means connecting said resistor to a high voltage power supply, a material storage reservoir, an electrically nonconductive conduit connecting said reservoir to said body passage, and means to prevent the failure of said resistor when said system is used to spray electrically conductive materials, said resistor failure preventing means comprising electrically insulative mounting means for said reservoir, electrically nonconductive means surrounding and encasing said reservoir to prevent electrical discharge of said reservoir when said system is utilized to spray electrically conductive materials, and means to ground said reservoir when said system is used to spray electrically non-conductive materials. 